1. Field of the Invention
This invention relates to a surface acoustic wave device (SAW device) which employs a piezoelectric thin film, and more particularly to a SAW device which is high in stability and easy to produce for realizing a narrowband timing tank filter or bandpass filter for use in a microwave region of several GHz to 20 GHz or a wideband voltage-controlled oscillator or a wide bandpass filter which is used in a wide frequency range from the VHF-band to the UHF-band.
2. Description of the Related Art
There is a tendency that transinformation content progressively increases as optical communication spreads, and the frequency of transmission signals is being expanded to a microwave region. At present, the development of ultra-high speed optical communication systems of several GHz to 20 GHz-band is in progress. However, since highly stable timing tank filters or bandpass filters for several GHz to 20 GHz-band are necessary to optical repeaters for several GHz to 20 GHz-band, it is a current subject to realize them with SAW devices.
Since generally the finger width d of interdigital transducer electrodes (IDT electrodes) employed in SAW devices and the wave length .lambda. of a surface acoustic wave (SAW) have a relationship of d=.lambda./4, the phase velocity V.sub.P, the frequency f of the propagating SAW and the finger width d of the IDT electrodes have a relationship of f=V.sub.P /4d.
When IDT electrodes are to be formed making use of the electron beam or the photo-lithography techniques wherein far-ultraviolet radiation is employed as a light source, since there is a limitation in reduction of the finger width d of IDT electrodes, it is necessary to raise the phase velocity V.sub.P of the SAW in order to realize SAW devices wherein the frequency f ranges from several GHz to 20 GHz.
In recent years, many investigations and many proposals have been made to realize GHz-band SAW filters, however, only 1.8 GHz-band SAW filters have been put into practical use wherein the phase velocity (propagation velocity) V.sub.P of SAWs is about 5,000 m/sec. For such conventional 1.8 GHz-band SAW filter, since the finger width d (d=.lambda./4) of IDT electrodes can be about 0.7 .mu.m, an electrode pattern can be formed using the photo-lithographic techniques wherein far-ultraviolet radiation is employed as a light source.
However, if it is intended to realize, for example, a 7 GHz SAW filter with the structure of such conventional SAW filter, it is necessary to make the finger width d of the IDT electrodes smaller than a minimum value of 0.35 .mu.m which can be attained by the photo-lithography based on far ultraviolet exposure. Accordingly, it cannot be realized by such technique.
In short, since the minimum finger width d of IDT electrodes which can be realized by the present photolithography technique base on far-ultraviolet exposure is about 0.35 .mu.m, in order to realize a SAW filter for 7 GHz wherein the IDT electrodes can be formed using the above-mentioned technique, it is necessary to make the propagation velocity (phase velocity) V.sub.P of a SAW equal to about 10,000 m/sec at least (V.sub.P =7 GHz.times.4d.gtoreq.9,800 m/sec=10,000 m/sec).
On the other hand, in order to realize a narrowband SAW filter (loaded Q.sub.L =1,000) for use as a timing tank filter for 7 GHz-band, it is necessary to set the effective electromechanical coupling coefficient K.sup.2.sub.eff of SAWs to a value greater than or equal to 0.1%, and preferably the effective electromechanical coupling coefficient K.sup.2.sub.eff is set to a value greater than or equal to that of SAW in ST cut quartz, that is, is 0.15% or more (K.sup.2.sub.eff .gtoreq.0.15%).
Meanwhile, if it is considered that a plane (100) or plane (110) Si single-crystal, a plane (100) GaAs single-crystal or the like is used for a substrate of an LSI (large scale integrated circuit), in order to enable SAW devices to be integrated with LSIs therearound, it is necessary to realize the SAW devices constructed directly on a single-crystal semiconductor substrate, for example, a Si single-crystal, a GaAS single-crystal or the like.
However, since a Si single-crystal is a non-piezoelectric crystal of the cubic system and a GaAs single-crystal is a weak piezoelectric crystal of the cubic system, even if IDT electrodes are provided directly on a single-crystal semiconductor substrate, that is, a Si single-crystal or a GaAs single-crystal, the SAW will not be excited or will be excited but very weakly. Therefore, it is desirable to realize a SAW device of a composite substrate structure which includes at least a piezoelectric thin film and a single-crystal semiconductor substrate, that is, a Si single-crystal or a GaAs single-crystal.
Taking a summary of the foregoing aims, the characteristics required for a SAW device with which a narrowband timing tank filter or bandpass filter for use in a microwave region of several GHz to 20 GHz is realized are:
(1) V.sub.P .gtoreq.10,000 m/sec; and PA0 (2) K.sup.2.sub.eff .gtoreq.0.15%. PA0 (1) V.sub.P .gtoreq.8,000 m/sec; and PA0 (2) K.sup.2.sub.eff .gtoreq.0.5%.
Further, the requirement in integration of such SAW device with an LSI is that the SAW device has a composite substrate structure which includes at least a single-crystal semiconductor substrate and a piezoelectric thin film.
By the way, in order to realize wideband voltage controlled oscillator, a wide bandpass filter or the like over a wide frequency range from the VHF-band to the UHF-band using SAW devices, it is necessary to select piezoelectric materials having a high effective electromechanical coupling coefficient K.sup.2.sub.eff (K.sup.2.sub.eff .gtoreq.0.5% as an aim).
Meanwhile, in order to produce such SAW devices at a low cost, it is necessary to make it possible to form IDT electrodes by the ordinary photo-lithography. technique wherein an ultraviolet exposure device is employed as a light source. To this end, it is necessary for the IDT electrodes to have a finger width d (d=.lambda./4) greater than or equal to 1.0 .mu.m which is a possible minimum value in the case of the ordinary technique. In this instance, in order to realize SAW devices over a wide high frequency range from the VHF-band (100 MHz or so) to the UHF-band (2.0 GHz or so), it is necessary for the SAW to have a propagation velocity (phase velocity) V.sub.P higher than or equal to nearly 8,000 m/sec (V.sub.P =2.0 GHz.times.4d=8,000 m/sec).
Further, in recent years, as peripheral circuits of such SAW devices are LSIs, it is demanded for SAW devices to be integrated with LSIs.
Taking a summary of the foregoing aims, the characteristics required for SAW devices with which a wideband voltage controlled oscillator, a wide bandpass filter or the like for use over a wide frequency region from the VHF-band to the UHF-band is realized are:
Further, the requirement in integration of such SAW device with LSIs is that the SAW device has a composite substrate structure which includes at least a single-crystal semiconductor substrate and a piezoelectric thin film.